1. Field of the Invention
The present invention relates to inhibitors of protein kinases. The invention also provides pharmaceutical compositions comprising the compounds of the invention and methods of using the compositions in the treatment of various diseases.
2. Background of the Invention
Protein kinases constitute a large family of structurally related enzymes that effect the transfer of a phosphate group from a nucleoside triphosphate to a protein acceptor. A vast array of cellular functions, including DNA replication, cell cycle progression, energy metabolism, and cell growth and differentiation, are regulated by reversible protein phosphorylation events mediated by protein kinases. Additionally, protein kinase activity has been implicated in a number of disease states. Accordingly, protein kinase targets have attracted substantial drug discovery efforts in recent years, with several protein kinase inhibitors achieving regulatory approval (reviewed in Fischer, Curr. Med. Chem., 11:1563 (2004); Dancey and Sausville, Nature Rev. Drug Disc., 2:296 (2003)).
Mitosis is a stage in the cell cycle during which a series of complex events ensure the fidelity of chromosome separation into two daughter cells. Several current cancer therapies, including the taxanes and vinca alkaloids, act to inhibit the mitotic machinery. Mitotic progression is largely regulated by proteolysis and by phosphorylation events that are mediated by mitotic kinases. Aurora kinase family members (e.g., Aurora A, Aurora B, Aurora C) regulate mitotic progression through modulation of centrosome separation, spindle dynamics, spindle assembly checkpoint, chromosome alignment, and cytokinesis (Dutertre et al., Oncogene, 21: 6175 (2002); Berdnik et al., Curr. Biol., 12: 640 (2002)). Overexpression and/or amplification of Aurora kinases have been linked to oncogenesis in several tumor types including those of colon and breast (Warner et al., Mol. Cancer Ther., 2: 589 (2003); Bischoff et al., EMBO, 17: 3062 (1998); Sen et al., Cancer Res., 94: 1320 (2002)). Moreover, Aurora kinase inhibition in tumor cells results in mitotic arrest and apoptosis, suggesting that these kinases are important targets for cancer therapy (Ditchfield, J. Cell Biol., 161: 267 (2003); Harrington et al., Nature Med., 1 (2004)). Given the central role of mitosis in the progression of virtually all malignancies, inhibitors of the Aurora kinases are expected to have application across a broad range of human tumors.
PLK is a serine/threonine protein kinase that plays a key role in cell cycle control. PLK controls entry and progression through mitosis at multiple stages by regulating centrosome maturation, activation of initiating factors, degradation of inhibitory components, chromosome condensation, and exit from mitosis (reviewed in Barr et al., Nature Reviews Mol Cell Biol., 5; 429 (2004)). PLK has been reported to be overexpressed in numerous cancers such as melanoma, ovarian, colorectal, lung and squamous cell carcinoma of the head and neck. Increased levels of expression are additionally correlated with poor prognosis and survival. (Kneisel et al., J Cutan Pathol 29: 354 (2002); Takai et al. Cancer Lett 164: 41 (2001); Takahashi et al Cancer Sci.; 94(2):148 (2003); Wolf et al. Oncogene 14: 543 (1997); Knecht et al. Cancer Res. 59 (1999)). Overexpression of the kinase transforms cells, rendering them oncogenic such that they acquire the ability to form tumors in mice (Smith et al., Biochem. Biophys. Res. Commun. 234; 397 (1997)). PLK protein levels are also elevated in tumor relative to normal cell lines in culture. Downregulation of PLK protein expression by RNA interference in tumor cell lines results in a reduction of cell proliferation, mitotic arrest at prometaphase and the rapid progression into apoptosis (Spankuch-Schmitt et al. J. Natl. Cancer Inst. 94(24):1863 (2002); Spankuch-Schmitt et al. Oncogene 21(20):3162 (2002)). This effect was not observed in normal cell lines. Moreover downregulation of PLK by short hairpin expression in mice with human xenografts reduced tumor growth to 18% (Spankuch B et al. (2004) J. Natl. Cancer Inst. 96(11):862-72). The key role of PLK in mitotic progression, its overexpression in a wide range of malignancies and the anti-proliferative effect observed upon its inhibition demonstrate its feasibility as a therapeutic target.
Anti-mitotic drugs that bind to tubulin (taxanes and vinca alkaloids) are currently utilized as chemotherapeutic drugs. Tubulin regulates cellular processes outside of mitosis therefore these drugs offer no selectivity towards cancer cells and are toxic to normal cells. Small molecule inhibitors that target the mitotic process specifically by targeting kinases that are overexpressed and active only in rapidly proliferating mitotic cells, such as PLK and Aurora kinase, may be clinically effective against tumors and not constrained by dose-limiting toxicities.
Cell cycle checkpoints are regulatory pathways that control the order and timing of cell cycle transitions. They ensure that critical events such as DNA replication and chromosome segregation are completed in high fidelity. The regulation of these cell cycle checkpoints is a critical determinant of the manner in which tumor cells respond to many chemotherapies and radiation. Many effective cancer therapies work by causing DNA damage; however, resistance to these agents remains a significant limitation in the treatment of cancer. One important mechanism leading to drug resistance is the activation of a checkpoint pathway that arrests the cell cycle to provide time for repair and induces the transcription of genes that facilitate repair. Cell cycle progression is prevented, and immediate cell death of the damaged cell is avoided. By abrogating such checkpoint arrests at, for example, the G2 checkpoint, it may be possible to synergistically augment the tumor cell death induced by DNA damage and to circumvent resistance. (Shyjan et al., U.S. Pat. No. 6,723,498 (2004)). Human Chk-1 plays a role in regulating cell cycle arrest by phosphorylating the phosphatase cdc25 on Serine 216, thereby preventing the activation of cdc2/cyclin B and the initiation of mitosis. (Sanchez et al., Science, 277:1497 (1997)). Therefore, inhibition of Chk-1 should enhance the effect of DNA damaging agents by initiating mitosis before DNA repair is complete, thereby promoting tumor cell death.
Accordingly, there is a need to develop inhibitors of protein kinases, including Chk-1, Aurora, and PLK. Such inhibitors are useful for treating various diseases or conditions associated with protein kinase activity, and are especially needed in view of the inadequate treatments currently available for many of these disorders.